Research in polymer electrolytes for use in rechargeable lithium and lithium-ion batteries has been ongoing for decades. However, performance of conventional electrolytes is still less than desirable. A long-standing goal of electrolyte research generally relates to the preparation of an electrolyte that combines the solid-like characteristics of conventional thermoplastics with the ionic conductivity of low molar mass liquids.
The majority of polymer electrolyte research has focused on employing polyethylene oxide (PEO) and PEO derivatives complexed with various lithium salts. A lithium salt dissolved in a high-molecular weight PEO by itself typically does not yield a room temperature conductivity sufficiently high enough for practical application in a lithium battery (i.e., greater than 10.sup.-3 S/cm). Various approaches have been attempted to develop solid electrolytes for lithium cells, which yield improved room temperature conductivity. Fauteux et al. [Electrochimica Acta, 1995, 40, 2185] has categorized these approaches into two classes, namely (1) "pure" solid polymer electrolyte systems, and (2) network or gel-polymer electrolyte systems, and Koksbang et al. [Solid State Ionics, 1989, 36, 320] has compiled examples of each class. In the first class, blends of polyethers which may be copolymerized with co-monomers can be used to lower the glass transition temperature T.sub.g and produce a polymer with reduced crystallinity. The class also includes co-polymers of polyether-grafted polyether, polysiloxane, or polyphosphazene backbones. Alternatively, plasticizers may be added to the PEO, either as an absorbed liquid or a plasticizing lithium salt, to attempt to effect an increase in conductivity.
In contrast, with respect to the gel-polymer electrolyte systems, the ionic conduction is believed to be due to the mobility of the ionic species in the liquid phase of the gel-polymer system. Gel electrolytes are typically prepared by dispersing liquid solvents (e.g., organic carbonates) and/or platicizers in an inert polymer. The ionic conductivity of the gel electrolyte is usually higher than the solid electrolyte, but at the expense of a less mechanically strong and electrochemically less stable system.
There remains a need in the art for electrolytes for use in electrochemical cells including lithium and lithium-ion batteries, which exhibit high conductivities and good mechanical properties. There is also a need for electrolytes which are more easily processable than conventional electrolytes. Moreover, there is a need for electrolytes which exhibit a higher level of electrochemical stability than those materials which are currently being used.
It is therefore an object of the present invention to provide a composite electrolyte which exhibits a high conductivity at room temperature. It is also an object of the present invention to provide a composite electrolyte which possesses good mechanical properties. It is another object of the present invention to provide a composite electrolyte which may be processed in an easier manner. It is yet another object of the present invention to provide a composite electrolyte which exhibits good electrochemical stability.